In many situations, it is desirable to remove a surface coating from the substrate to which it is adhered, for reasons such as repair, repainting or inspection of the substrate. There are many instances in which such removal becomes problematic, such as when the substrate is particularly susceptible to damage as with thin substrates and substrates made of composite materials.
In particular, in the aircraft industry, removal of surface coatings is significantly difficult. The surfaces of aircraft are typically very thin, on the order of 0.020 inches thick, and may be made of composite materials so as to reduce the weight while maintaining high strength structures. Although composite materials are not susceptible to corrosion or fatigue cracking, metal air frames must be treated for corrosion and inspected periodically to prevent catastrophic failure due to metal fatigue. Surface coatings must be completely removed in order to conduct a thorough inspection. During maintenance operations, all aircraft surfaces and components must typically be thoroughly cleaned. The process used to remove a surface coating from an aircraft surface or component must not cause damage thereto. At the same time, the process must be capable of completely removing the surface coating.
Presently, chemicals are typically used to remove surface coatings from aircraft. These chemical compounds frequently are ineffective and inefficient, requiring several applications and manual scrubbing of the surfaces. These chemicals are generally highly toxic, and dangerous to use. Although protective clothing is available, it is frequently not used because it is uncomfortable, hot and interferes with the efficiency of the cleaning process.
The use of chemicals to clean aircraft present problems to the environment of the worker as well as to the earth's environment. The chemicals are preferably used in an enclosed area so that the fumes and airborne constituents of the chemicals and surface coating may be filtered and prevented from release to the atmosphere. However, because of the size of aircraft, no matter what precautions are taken, some chemicals may leak into the atmosphere. There is a disposal problem with the chemicals as well, which must be treated as hazardous waste.
Media blasting has also been used in an attempt to remove such surface coatings. One such example is plastic media blasting (PMB), which has met with only limited success. The removal of the surface coating utilizing only the kinetic energy of the plastic media and thereby abrading the coating requires that the particles impart sufficient energy to the coating. At the energy levels necessary to remove the coating, some damage to the substrate is typically inevitable. The plastic media also tends to become lodged in structural joints and other areas. Although the plastic media is reusable, the efficiency of the PMB process drops by about 75% when the media is reused, even in combination with new media. Even though PMB does not produce hazardous waste as chemicals do, the used plastic media is contaminated with the removed surface coating and large quantities of media must be disposed of.
Cryogenic particle blasting, and as more specifically described herein, CO.sub.2 particle blasting, has also been used to remove surface coatings from aircraft surfaces and components. Because the CO.sub.2 pellets sublimate into a gas which is naturally found in the atmosphere, cleanup and environmental concerns are minimized. Even though CO.sub.2 pellets may become lodged in structural joints, the characteristic of sublimation causes this to be inconsequential. However, CO.sub.2 particle blasting may be too slow for the removal of some coatings, and may be too aggressive to be used on certain substrates.
Equipment and methods relating to CO.sub.2 particle blasting are disclosed in U.S. Pat. Nos. 4,744,181, 4,843,770, 4,947,592, 5,018,667, 5,050,805 and 5,063,015, all of which are incorporated herein by reference. As used herein, it will be understood that CO.sub.2 particle blasting refers not only to the blasting process which utilizes carbon dioxide pellets or particles, but any cryogenic particle blasting process which utilizes sublimable pellets or particles.
Another way to remove surface coatings is to ablate the surface coating by heating the surface coating above its chemical flash point temperature so that it is ablated. The surface coatings can be heated very quickly to such temperatures by impinging the surface coating with photon energy. Sources of photon energy include lasers, such as CO.sub.2 lasers, ruby lasers and xenon lasers. Once the surface coating is completely ablated, the residue must be removed. Chemical compounds as well as CO.sub.2 particle blasting have been used to remove this residue after the ablation process is complete.
Ablation of surface coatings presents problems with heat damage to the substrate. If the incident photon energy is applied for too long a period of time, significant heat will transfer to the substrate, raising its temperature and damaging it. If there is also a surface coating on the backside of the substrate, which is frequently is inaccessible, that surface coating may peel due to the increased temperature of the substrate, and expose the backside of the substrate to corrosive conditions.
Therefore, the use of lasers to ablate a surface coating requires substantial control of the process. For example, with a monofrequency laser such as a CO.sub.2 laser, a continuously moving beam is swept across the area of impingement of the surface coating. The sweep rate of the beam is one way to control how much energy is imparted to a specific location within the area of impingement. Thus, any particular location is impinged by the relatively narrow beam several times for a short duration, as the area of impingement advances across the surface coating. The laser beam itself may be a continuous beam or it may be pulsed. In either case, specific locations on the surface coating are directly impinged by the beam several times for a short duration.
Although it is possible to provide adequate beam control in a laboratory setting so as to ablate a surface coating to a controlled depth, when applied to the removal of a surface coating on an aircraft there are substantial problems. Because the laser is powerful enough to damage the metal substrate, if the operator allows the area of impingement to dwell at one place for too long, if the standoff distance varies too much, or if the thickness of the surface coating varies, such as from 0.008 inches to 0.004 inches, the laser can completely ablate the surface coating and impinge directly on the substrate, thereby damaging it. Sufficient beam control has not yet been achieved to allow the use of lasers to ablate surface coatings on aircraft surfaces and components.
Another type of laser utilizing xenon has also been used to ablate surface coatings on aircraft. Xenon lasers, referred to generically herein as "flashlamps" are known in the art and have been described, for example, in U.S. Pat. Nos. 4,075,579, 4,450,568, 4,837,794, 4,867,796, 4,871,559, 4,910,942, 4,975,918 and 5,034,235, all of which are incorporated herein by reference. The flashlamp consists of a quartz tube filled with xenon gas which emits a brilliant flash of light when electrically energized. This light is multifrequency. The impingement of this photon energy on surface coatings results in the ablation of the coating. However, its usefulness with respect to aircraft surfaces and components is limited because of the heat transfer to the substrate. Additionally, when the outer surface of the coating is ablated, it becomes charred, and if left in place impedes the penetration of subsequent photon energy flashes from the flashlamp, preventing the ablation of the entire thickness of the coating.
Thus, there remains a need for an efficient and cost effective process which is capable of completely removing a surface coating from a substrate, such as aircraft surfaces and components, without damaging the substrate. The process must avoid the use of hazardous materials and disposal requirements of any materials used.